Related U.S. Patents assigned to the assignee of the present invention include: U.S. Pat. Nos. 5,521,601; 5,528,222; 5,538,803; 5,550,547; 5,552,778; 5,554,974; 5,563,583; 5,565,847; 5,606,323; 5,635,693; 5,673,037; 5,680,106; 5,682,143; 5,729,201; 5,729,697; 5,736,929; 5,739,754; 5,767,789; 5,777,561; 5,786,626; 5,812,065; and 5,821,859. U.S. Patent applications assigned to the assignee of the present invention include: application Ser. No. 08/626,820, filed: Apr. 3, 1996, (now U.S. Pat. No. 5,850,181 issued Dec. 15, 1998) entitled xe2x80x9cMethod of Transporting RF Power to Energize Radio Frequency Transpondersxe2x80x9d, by Heinrich, Zai, et al.; application Ser. No. 08/694,606 filed Aug. 9, 1996 (now U.S. Pat. No. 5,942,987 issued Aug. 24, 1999); Radio frequency identification system with write broadcast capability entitled RFID System with Write Broadcast Capability by Cesar et al.; application Ser. No. 08/681,741 filed Jul. 29, 1996 (now U.S. Pat. No. 5,874,902 issued Feb. 23, 1999) entitled RFID Transponder with Electronic Circuitry Enabling and Disabling Capability, by Heinrich, Goldman et al.; and application Ser. No. 09/153,617 filed Sep. 15, 1998, (now U.S. Pat. No. 6,122,329 issued Sep. 19, 2000) entitled RFID Interrogator Signal Processing System for Reading Moving Transponder, by Zai et al. The above identified U.S. Patents and U.S. Patent applications are hereby incorporated by reference.
1. Field of the Invention
The field of the invention is the field of Radio Frequency (RF) transmission of power to supply energy to remote electronic equipment, especially equipment for the location, identification, and measurement of objects, items, animals, or people associated with RF transponders.
2. Description of the Prior Art
RF Transponders (RF Tags) can be used in a multiplicity of ways for locating and identifying accompanying objects and transmitting information about the state of the object. It has been known since the early 60""s in U.S. Pat. No. 3,098,971 by R. M. Richardson, that electronic components of transponders could be powered by radio frequency (RF) electromagnetic (EM) waves sent by a xe2x80x9cbase stationxe2x80x9d and received by a tag antenna on the transponder. The RF EM field induces an alternating current in the transponder antenna which can be rectified by an RF diode on the transponder, and the rectified current can be used for a power supply for the electronic components of the transponder. The transponder antenna loading is changed by something that was to be measured, for example a microphone resistance in the cited patent. The oscillating current induced in the transponder antenna from the incoming RF energy would thus be changed, and the change in the oscillating current led to a change in the RF power radiated from the transponder antenna. This change in the radiated power from the transponder antenna could be picked up by the base station antenna and thus the microphone would in effect broadcast power without itself having a self contained power supply. The xe2x80x9crebroadcastxe2x80x9d of the incoming RF energy is conventionally called xe2x80x9cback scatteringxe2x80x9d, even though the transponder broadcasts the energy in a pattern determined solely by the transponder antenna. Since this type of transponder carries no power supply of its own, it is called a xe2x80x9cpassivexe2x80x9d transponder to distinguish it from a transponder containing a battery or other energy supply, conventionally called an active transponder.
Active transponders with batteries or other independent energy storage and supply means such as fuel cells, solar cells, radioactive energy sources etc. can carry enough energy to energize logic, memory, and tag antenna control circuits. However, the usual problems with life and expense limit the usefulness of such transponders.
In the 70""s, suggestions to use backscatter transponders with memories were made. In this way, the transponder could not only be used to measure some characteristic, for example the temperature of an animal in U.S. Pat. No. 4,075,632 to Baldwin et. al., but could also identify the animal.
The continuing march of semiconductor technology to smaller, faster, and less power hungry has allowed enormous increases of function and enormous drop of cost of such transponders. Presently available research and development technology will also allow new function and different products in communications technology. However, the new functions allowed and desired consume more and more power, even though the individual components consume less power.
It is thus of increasing importance to be able to power the transponders adequately and increase the range which at which they can be used. One method of powering the transponders suggested is to send information back and forth to the transponder using normal RF techniques and to transport power by some means other than the RF power at the communications frequency. However, such means require use of possibly two tag antennas or more complicated electronics.
Sending a swept frequency to a transponder was suggested in U.S. Pat. No. 3,774,205. The transponder would have elements resonant at different frequencies connected to the tag antenna, so that when the frequency swept over one of the resonances, the tag antenna response would change, and the backscattered signal could be picked up and the resonance pattern detected.
Prior art systems can interrogate the tags if more than one tag is in the field. U.S. Pat. No. 5,214,410, hereby incorporated by reference, teaches a method for a base station to communicate with a plurality of tags.
Sending at least two frequencies from at least two antennas to avoid the xe2x80x9cdead spotsxe2x80x9d caused by reflection of the RF was proposed in EPO 598 624 A1, by Marsh et al. The two frequencies would be transmitted simultaneously, so that a transponder in the xe2x80x9cdead spotxe2x80x9d of one frequency would never be without power and lose its memory of the preceding transaction.
The prior art teaches a method to interrogate a plurality of tags in the field of the base station. The tags are energized, and send a response signal at random times. If the base station can read a tag unimpeded by signals from other tags, the base station interrupts the interrogation signal, and the tag which is sending and has been identified, shuts down. The process continues until all tags in the field have been identified. If the number of possible tags in the field is large, this process can take a very long time. The average time between the random responses of the tags must be set very long so that there is a reasonable probability that a tag can communicate in a time window free of interference from the other tags.
In order that the prior art methods of communicating with a multiplicity of tags can be carried out, it is important that the tags continue to receive power for the tag electronics during the entire communication period. If the power reception is interrupted for a length of time which exceeds the energy storage time of the tag power supply, the tag xe2x80x9closesxe2x80x9d the memory that it was turned off from communication, and will restart trying to communicate with the base station, and interfere with the orderly communication between the base station and the multiplicity of tags.
The amount of power that can be broadcast in each RF band is severely limited by law and regulation to avoid interference between two users of the electromagnetic spectrum. For some particular RF bands, there are two limits on the power radiated. One limit is a limit on the continuously radiated power in a particular bandwidth, and another limit is a limit on peak power. The amount of power that can be pulsed in a particular frequency band for a short time is much higher than that which can be broadcast continuously.
Federal Communications Commission Regulation 15.247 and 15.249 of Apr. 25, 1989 (47 C.F.R. 15.247 and 15.249) regulates the communications transmissions on bands 902-928 MHZ, 2400-2483.5 MHZ, and 5725-5850 MHZ. In this section, intentional communications transmitters are allowed to communicate to a receiver by frequently changing frequencies on both the transmitter and the receiver in synchronism or by xe2x80x9cspreading outxe2x80x9d the power over a broader bandwidth. The receiver is, however, required to change the reception frequency in synchronism with the transmitter.
U.S. Pat. No. 5,408,202 by Shirazi, et al., issued Apr. 18, 1995, and U.S. Pat. No. 6,107,843 by de Gouy, et al., issued Aug. 22, 2000, give details on fractional phase lock loops which can control oscillators with a speed necessary for the purposes of the present invention.
An article by Marc Zuta, in Microwave Journal of June, 1998 (page 94) gives details on a representative feedback circuit which can control an oscillator with a speed necessary for the present invention.
It is an object of the invention to provide an improved system, apparatus, and method to transport RF power to an RF transponder.
It is further an object of the invention to transport RF power to an RF transponder by means of switching frequencies from a first frequency to a second frequency in a very short time.
It is further an object of the invention to transport RF power to an RF transponder by providing circuits which allow switching frequencies from a first frequency to a second frequency in a very short time.
The present invention is an apparatus, a system, and a method to use a xe2x80x9chopping frequencyxe2x80x9d signal to power remote electronic equipment such as RF transponders. In essence, an RF transmitter broadcasts a series of high power pulses, where the frequency of each pulse is chosen in order from a pseudo randomly ordered list of allowed frequencies. The transponders are able to receive power at all the frequencies sent. The energy received is stored in an energy store on the tag. The time between pulses is preferably shorter than the time taken for the tag electronics to
The time between pulses is preferably shorter than the time taken for the tag electronics to The transponder could be passive or active, and the power transported to an active
The transponder could be passive or active, and the power transported to an active when the transponder is in the range of the base station transmitter, and thus save the battery energy which would not be needed to xe2x80x9clisten forxe2x80x9d the communications when the transponder was not in the range of the transmitter of the base station.